Some of the most persistent contaminating substances consist in polychlorinated dibenzo-p-dioxins, dibenzofurans and dioxin-like compounds. These are chemically stable substances that are very difficult to remove from the environment by chemical, thermal and biological procedures. These compounds are toxic and are classified amongst teratogenic and carcinogenic substances. They are formed in thermal processes, e.g. in the combustion of municipal, hospital and other hazardous wastes, in metallurgical processes and in the use of a number of other thermal technologies, or are manufactured for applications in the energy industry, agriculture and other branches.
Of the techniques employed to date for the destruction of these toxic substances, especially the reaction of these compounds with sodium of alkali metal alkoxides, as described in EP 1 153 645, is employed. Chemical decomposition, described in EP-A-0 021 294, is based on the reaction of halogenated aromatic substances with an alkali metal or with a mixture of alcohol with an alkali metal hydroxide, or with an alkali metal carbonate at a temperature of 140 to 220° C. Alkaline decomposition of polychlorinated biphenyls by sodium carbonate occurs at a temperature of 370 to 400° C. in the presence of an oxidizing agent and catalyst consisting in ruthenium or platinum or palladium, as described in JP 11 253 795, U.S. Pat. No. 4,059,677, U.S. Pat. No. 4,065,543 and JP 10 087 519. According to U.S. Pat. No. 5,151,401, platinum on zinc aluminate can also be used. JP 11 114 538 describes the pressure decomposition of polychlorinated biphenyls and polyfluorinated dibenzo-p-dioxins by calcium hydroxide at a temperature of 100 to 300° C. Patent documents WO 00/48968 and JP 11 197 756 describe the catalytic reduction of polyfluorinated dibenzo-p-dioxins in alkaline medium in the presence of hydrazine thiosulphate, hydroquinone and titanium dioxide on a carbon support matrix, or in a medium of zinc hydroxide and carbonate or lead hydroxide and carbonate, occurring at a temperature of 200 to 500° C.
Thermal decomposition of halogenated aromatic compounds requires that a temperature of 1200 to 1400° C. be attained. However, this decomposition process is of uncertain significance, because considerations related to practical applications do not take into account reversible processes occurring in the gas phase and designated as denovo synthetic reactions, in which the pollutants are reformed from the precursors in the temperature range 180 to 450° C. on the solid phase.
It is advantageous if some metals are present in the thermal detoxication of halogenated aromatic compounds, for example, aluminium, iron and copper, or their oxides, or melted aluminium or aluminium, magnesium, silicon, titanium or beryllium in an inert atmosphere at a temperature of 450 to 650° C., as is apparent from patent documents JP 11 253 908, EP 0 170 714 and EP 0 184 342. U.S. Pat. No. 3,697,608 describes the use of a dechlorination agent consisting of ferrous chloride or ferric chloride with alkali metal chlorides in the melt.
Catalytic decomposition of halogenated aromatic substances is considered to be very promising for practical applications in liquidation of these toxic substances. Nonetheless, the above-described decomposition processes do not constitute an optimal approach for dehalogenation detoxication of halogenated aromatic compounds, as these chemical processes are expensive, dangerous in the case of use of the sodium method and, for the combustion method, have high energy consumption and are not efficient because of the denovo synthetic reverse reactions.
EP-A-0 184 342 describes the use of metal catalysts to decompose organic halogenated compounds, e.g. polychlorinated biphenyls, in the gas phase at a temperature of 450 to 650° C. in a strictly nonoxidizing very pure nitrogen or rare gas atmosphere. Patent document JP 11 904 460 describes the use of metal hydride and palladium on a carbon matrix for detoxication of organic halogenated aromatic compounds. U.S. Pat. No. 4,039,623 describes the oxidation catalytic decomposition of halogenated compounds at a temperature of 350° C. catalyzed by ruthenium. According to Organohalogen Compounds 40, 583-590 (1999), polychlorinated biphenyls can be decomposed at a temperature of 150 to 300° C. using the TiO2—V2O5—WO3 catalytic system. Patent documents U.S. Pat. Nos. 3,972,979 and 3,989,806 describe the catalytic dehalogenation of hexachlorobenzene at a temperature of 500° C. using a catalyst consisting of copper on zeolite or chromium (III) oxide on a support. EP 0 914 877 and U.S. Pat. No. 6,291,737 describe the decomposition of dioxins in the presence of amines or ammonium salts at a temperature below 300° C. Patents U.S. Pat. Nos. 5,276,250 and 5,387,734 describe the dehalogenation of compounds in an inert atmosphere using catalysts containing calcium, barium, zinc, nickel, copper, iron, aluminium, palladium, platinum, vanadium, tungsten, molybdenum, rhodium and chromium, sometimes in the form of oxides, silicates or aluminates, with a mass ratio of the catalyst to the dehalogenated substance of 1:1 to 1:30 and at a temperature of 150 to 550° C. The article by Pekárek V. et al., ESPR-Environ. Sci. and Pollut. Res. 10(1), 39-43 (2003) describes a system for dehalogenation of ash from incineration of municipal waste using a combination of copper and carbon.
All the above dehalogenation procedures have the disadvantage that they have high energy demands and/or do not lead to complete detoxication of the dehalogenated material and/or do not constitute a closed, risk-free waste-free cycle.